1. Field of the Invention
The present invention relates to a power conversion apparatus, and in particular to a power conversion apparatus for realizing a high output power density converter configured with a plurality of switches or a power integrated circuit.
2. Description of the Related Art
FIGS. 2 to 5 are diagrams showing the configuration of a conventional two-level power conversion circuit. FIG. 2 shows a three-phase inverter, FIG. 3 a single-phase inverter, FIG. 4 a DC-DC converter, and FIG. 5 a three-phase/three-phase power converter. In each case, the DC power source voltage Vdc is switched in two levels by turning on/off six, four, or two semiconductor elements, and further, by connecting a passive filter to the output thereof, a three-phase or single-phase AC output or DC output is obtained. A power converter conventionally achieves a high output power density with this two-level power conversion circuit by reducing the volume according to a method (1) in which the loss of the power conversion apparatus is reduced, thereby reducing the volume of the cooling unit, or (2) in which the switching frequency is increased, thereby reducing the volume of the passive parts such as the passive filter configured with an inductor or capacitor (see Y. Hayashi, K. Takao, K. Adachi, and H. Ohashi, “Design Consideration for High Output Power Density (OPD) Converter Based on Power-Loss Limit Analysis Method”, in Proc. CD-ROM, EPE, 2005, and M. Tsukuda, T. Omura, W. Saito, and T. Ogura, “Demonstration of High Output Power Density (30 W/cc) Converter using 600 V SiC-SBD and Low Impedance Gate Driver”, in Proc. CD-ROM, IPEC Niigata, 2005.). However, because the two-level power conversion circuit always contains a high harmonic, the passive filter is indispensable. Also, the increased switching frequency increases the switching loss at the time of the switching operation of the semiconductor elements, resulting in a bulky cooling unit; therefore, the increase in output power density is limited.
In cases where the power conversion apparatus is driven with a high switching frequency, the induction voltage due to the parasitic inductance of a main circuit wiring and the displacement current due to the parasitic capacitance cause electromagnetic noise and an increased loss of the semiconductor element. Electromagnetic noise gives rise to various problems such as the malfunction of the gate drive circuit, the deteriorated insulation of a motor connected to the power converter or the electric erosion of a bearing. Thus, a power conversion circuit of high output power density is required from which the passive filter is eliminated and which reduces the loss of the semiconductor element at the same time without increasing the switching frequency.
In cases where the power conversion apparatus is driven with a high switching frequency, the common mode voltage of the power conversion circuit vibrates, causing peripheral devices to be adversely affected. For this reason, a noise suppression filter such as a common mode choke coil or EMI filter is required.
In order to remove the passive filter without increasing the switching frequency, a method is available in which, as shown in FIGS. 6 to 8, the number m of the levels of the multilevel power converter is increased, therefore decreasing the harmonic content of the output voltage of the power converter (Japanese Patent Application Laid-Open No. 2007-325480). The high harmonic component of the output voltage of the multilevel power converter decreases with the increase in the number of levels. For example, the total distortion rate of the output phase voltage is 5% or lower for 17 levels, 3% or lower for 25 levels, and 2% or lower for 35 levels. This shows that the passive filter becomes unnecessary as the number m of levels increases. The number of semiconductor elements is 32 and the number of inverse parallel diodes is 32 per phase for the 17-level power converter; the number of semiconductor elements is 48 and the number of inverse parallel diodes is 48 per phase for the 25-level power converter; and the number of semiconductor elements is 68 and the number of inverse parallel diodes is 68 per phase for the 35-level power converter. As a result, the gate drive circuit connected to the main circuit switch and the main circuit of the power converter increases in size to such an extent that it becomes difficult to implement the power converter.
FIGS. 6 to 8 show representative systems of the multilevel power conversion circuit. FIG. 6 shows a three-phase inverter of a diode-clamp multilevel power converter, FIG. 7 a three-phase inverter of a flying-capacitor multilevel power converter, and FIG. 8 a three-phase inverter of a cascade-connected multilevel power converter. The three-phase inverter shown in each case may be a three-phase AC-DC power converter, a single-phase inverter, a single-phase AC-DC converter, a DC-DC converter, or an AC-AC converter.
The multilevel power conversion circuits shown in FIGS. 6 to 8 require a gate drive circuit for each switching semiconductor element; therefore, the number of gate drive circuits increases as the number of levels increases. In the m-level power conversion circuit shown in FIGS. 6 to 8, for example, (2m−2) gate drive circuits are required for each phase.
FIG. 9 is a diagram showing the configuration of one phase of the conventional multilevel power conversion circuit that includes switches connected in a series, and gate drivers and dedicated power supplies connected to the respective switches. The potential on the low potential side of each switch connected in the series differs from one switch to another; therefore, each dedicated power supply requires insulation. As a result, a transformer or similar device is used as the dedicated power supply, thereby making integration difficult.
A one-chip power IC and an intelligent power module (IPM) using LSI technology have been developed and have found applications in various fields. Although an integrated two-level power converter with a gate drive circuit using this technology has been proposed, a power integrated circuit that includes an insulated power supply with increased number of gate drive circuits in the multilevel power converter has never been proposed.
In the high output power density conversion apparatus using the two-level power converter described above, the filter is reduced in size by increasing the switching frequency; therefore, the switching loss is increased at the time of the switching operation of the semiconductor element, thereby limiting the higher output power density of the power conversion apparatus.
In cases where the power conversion apparatus is driven by a high switching frequency, a great amount of electromagnetic noise is generated by the induced voltage due to the parasitic induction of the main circuit wiring, the displacement current due to the parasitic capacitance and the vibration of a common mode voltage of a power conversion circuit. As a result, a noise suppression filter such as a common mode choke coil or EMI filter is required, which in turn limits the higher output power density of the power conversion apparatus.
The great amount of electromagnetic noise generated from the power conversion apparatus driven by a high switching frequency makes it impossible to realize a high-speed rotation of a motor connected to the power conversion apparatus, and thereby limits a higher output power density of the motor.
There has been a proposal for realization of a high output power density converter by a very different method from that described above: reducing the size of the passive filter without increasing the switching frequency (Japanese Patent Application Laid-Open No. 2007-325480). Nevertheless, there has been no proposal for a high output power density converter or a power integrated circuit for a multilevel power conversion circuit that includes an insulated power supply of a gate drive circuit.
Because a dedicated power supply configured with a transformer or similar device is required for the insulated power supply of the gate drive circuit, a power conversion apparatus with a number of switches that have an output power capacity of several dozen of kVA or less has not been realized.
Because a dedicated power supply configured with a transformer or similar device for the insulated power supply of the gate drive circuit is required, a power conversion apparatus with a phase voltage distortion rate of 10% or less without a passive filter for several dozen of kVA or less in the output power capacity has not been realized.
Also, because a dedicated power supply configured with a transformer or similar device is required for the insulated power supply of the gate drive circuit, a power conversion apparatus in which the electromagnetic noise has no influence on the peripheral devices without a noise suppression filter for an output power capacity of several dozen kVA or less has not been realized.